Implement KMS driver for Cirrus cards

by Leo Grange for The OpenBSD Foundation

With the current progress done on the graphic stack on many operating systems, the DRM/KMS infrastructure is now used by several BSDs, including OpenBSD. The aim of this project is to write a new KMS driver to handle a Cirrus CLGD 5446 card, and to document the process, to make writing new KMS driver for OpenBSD easier. In addition, the choice of this card will allow usage of KMS drivers when running OpenBSD through QEMU.